1. Field of the Invention
The present invention relates generally to an analog phase tracking circuit for use with interferometric fiber sensors and, more particularly, to an analog phase tracker for use with a fiber optic gyroscope for providing a linear output over a wide dynamic range (greater than .+-.500.degree. Sagnac phase shift) with good linearity and low noise and drift.
2. Description of the Related Art
Optical fiber based interferometric techniques have been investigated for use in rotation sensing for over a decade because of their usefulness in navigation applications. One of the chief goals of fiber optic gyroscope development is to isolate all environmental effects other than rotation from optical fiber sensors. A fiber gyroscope configuration which accomplishes this goal is the fiber Sagnac or ring interferometer, more commonly referred to as an interferometric fiber optic gyroscope.
There are two different types of Sagnac interferometers: open-loop and closed-loop gyroscopes. Both of these interferometer configurations produce a Sagnac effect and a corresponding Sagnac phase shift which is the basis for measuring rotation of optical fiber sensors.
One of the best advantages of using an open-loop gyroscope configuration is that for medium performance applications, such as short distance airline flights or missile guidance systems, the optics and demodulation technique are low in cost. Open-loop gyroscopes perform well in less-demanding applications, where the minimum detectable rotation may be approximately 1.degree. to 10.degree./hour and linearity approximately 0.1 to 1%. The disadvantages to using the open-loop gyroscope is its limited dynamic range and the need for scale factor parameter correction (a dynamic range of 3000 to 10,000 with a scale factor correction of 1 to 3% would be typical). Another disadvantage is that the scale factor which relates the output voltage to rotation rate depends directly on the source intensity and fringe visibility. Therefore the output has to be normalized to eliminate bias and scale facto drift.
In contrast to the open-loop gyroscopes, closed-loop gyroscopes are capable of measuring rotation rates with high linearity and wide dynamic range. Basically, the optical components of a closed-loop gyroscope introduce a nonreciprocal phase shift to compensate for the Sagnac phase shift caused by rotation. This is accomplished by phase shifting optically only one of the beams relative to the other beam to compensate or cancel the Sagnac phase shift. The magnitude of the Sagnac phase shift, even when multiplies of 2.pi. are considered, could be compensated by the closed-loop gyroscope. Limited dynamic range is irrelevant because a signal is being fed-back which compensates for the Sagnac phase shift. However, to modulate one beam and not the other requires very sophisticated and costly signal processing systems and optics, which is the greatest disadvantage of the closed-loop gyroscope.